1. Field of the Invention
This invention relates to a power supplying apparatus and method for use in an electrophotographic process, and in particular, relates to a power supplying apparatus and method capable of selectively supplying a constant current in the electrophotographic process, or stopping supplying of the constant current and instead impressing a prescribed bias voltage by modulating a duty cycle of a pulse generated by a switching regulator or the like.
2. Discussion of the Background
In a background image-forming apparatus such as a printer, a copier, a facsimile, and so on, which forms an image using an electrophotographic process, an electrical power source that supplies a high voltage of more than a few kilovolts to a discharge device so that the discharge device evenly discharges a surface of a photo-conductive drum (hereinafter referred to as a PC drum) with corona discharge is well known. Such an electrical power source generates a direct current to be supplied, for example, to a discharge device. The direct current is obtained by rectifying an alternating current induced by a transformer (as discussed below). The background electrical power source generally includes a switching regulator that outputs pulses of an electrical current having a prescribed duty cycle and a width and a switching element, such as a transistor, which is connected with the switching regulator to be activated and deactivated by the pulses output from the switching regulator.
The background electrical power source further includes a transformer connected with the switching element, which may generate a voltage as a primary voltage when the switching element is activated, and which obtains a step-up voltage as a secondary voltage. The background electrical power source further includes a rectifying-smoothing circuit connected with a secondary winding of the transformer, which may rectify and smooth an alternating current having the secondary voltage to obtain a direct current. The rectifying-smoothing circuit is connected with a discharge device for use, for example, in an image forming apparatus to supply the direct current thereto. A corotron type discharge device is generally used for the discharge device, which is generally constituted by a tungsten steel wire as an electrode and a shield plate.
To discharge a surface of a PC drum using the discharge device, a direct current having a voltage greater than a breakdown voltage of the discharge device is generally required to be supplied to the discharge device. To generate this voltage, the switching regulator is controlled to generate pulses of a direct current at a prescribed duty cycle to activate and deactivate the switching element.
In the background discharge device, toner or the like tends to be attracted by the tungsten wire or the like and to stick to the tungsten wire. This results because an electromagnetism remains in the tungsten wire due to a usage of a high voltage for the corona discharge or the like even after an electrical current is stopped from being supplied to the tungsten wire. As a result, the toner sticking on the tungsten wire may cause a contamination of an image when an image formation is executed with the discharge device. The background power supplying apparatus then impresses a bias voltage having a prescribed level to the discharge device during a time when corona is not discharged to the surface of the PC drum to prevent toner from sticking thereto.
However, the background power supplying apparatus requires a large space in the image forming apparatus and is costly. This results because an extra bias voltage-applying device that applies the bias voltage to the discharge device and a control device that controls the bias voltage-apply device are used in the background power supplying apparatus.